New phosphate remover marketing claim

[JasonLion]

I just saw an ad claiming that phosphates can cause "calcium-phosphate scale which can damage chlorine generator cells". It goes on to say that "Salt pools are especially vulnerable to problems from phosphates".

Obviously they are pushing phosphate remover. Still, it would be nice to know if there is any kind of truth behind this and develop a good explanation of why phosphates still don't really matter even in the face of this new claim.

 

[Guest]

Since the solubility of calcium carbonate is 14 mg/liter wile the solubility of calcium phosphate is 20 mg/liter the less souluable calcium carbonate,which is readily removed with acid, will precipitate first and lower the amount of calcium ins in solution which in turn would lower the propensity of calcium salts to precipitate out of solution. While it is true that calcium phosphate scale is very hard to remove the possibility of it forming in the first place is small in an over carbonated pool (in other words, normal TA levels) that has carbonate ions present in at least an order of magnetutde greater than phosphate ions.

A few years ago the lanthanum (phosphate remover) peddlers were saying that phosphates would keep a SWG from producidng chlorine by interfering with the reaction in the cell but would never back it up with the chemistry. This seems like their newest attempt at the same to increase their sales through fear.

 

[chem geek]

I agree with waterbear. Let me do some calculations, mostly for the case where the water isn't already saturated with calcium carbonate.

Calcium phosphate, Ca₃(PO₄)₂, has a solubility product of 1x10^-26. At a typical CH of 300 ppm (about 3x10^-3 moles/liter), this implies a phosphate saturation level of 6x10^-10. At a pH of 7.5, this implies a hydrogen phosphate, HPO₄^2-, of 9x10^-5 and a dihydrogen phosphate, H₂PO₄^-, of 4.6x10^-5 and a phosphoric acid, H₃PO₄, of 2x10^-10. So the total phosphate level is around 1.4x10^-4 or about 13 ppm or 13,000 ppb. This is far higher than would normally be found in pools. [EDIT] The solubility product of calcium phosphate is all over the map depending on source where a search I made found 1x10^-26, 2.07x10^-33, 2.83x10^-30, 1x10^-33, 2.0x10^-29. At 300 ppm CH with a pH of 7.5 this results in a total phosphate level at saturation of 8170 ppb, 3.7 ppb, 137 ppb, 2.6 ppb, 265 ppb, respectively. The 8170 ppb vs. 13,000 ppb difference is probably due to somewhat different dissociation constants for phosphoric acid when I originally made the calculation until now. [END-EDIT]

At higher pH, the same phosphate ion level (so same saturation level) implies a lower total phosphate level, so solubility decreases at higher pH. At a pH of 8.0, hydrogen phosphate is 2.8x10^-5, dihydrogen phosphate is 4.5x10^-6, phosphoric acid, 6x10^-12. So the total phosphate level is around 3.3x10^-5 or about 3 ppm or 3000 ppb. This is a level that could be found in pools and just means that high pH could precipitate calcium phosphate. Of course, it's also likely to precipitate calcium carbonate if the water is saturated as with plaster pools.

In an SWG cell at the cathode where hydrogen gas is generated, the pH gets high which is why scaling usually occurs at that plate. So it's possible to scale with calcium phosphate if the water were not saturated with calcium carbonate, though I would ask, "so what?". Many SWG cells change polarity every so many hours to remove scale that may have built up and calcium phosphate scale should dissolve in the acidic conditions of the (now) anode just as calcium carbonate would dissolve.

Richard

 

[budster]

Your above discussions are very helpful to me. I'd always believed that the reason Chemtura "po-po'd" phosphate removers was because they did not sell it. I've also heard, for many years, that phosphates are "algae food", so, removing phosphates helps in algae control.

I have not heard the original posted claims that phosphate removers are useful or critical for SWG pools. Thanks, again!

 

[Guest]

I've also heard, for many years, that phosphates are "algae food", so, removing phosphates helps in algae control.

While this is true so are Nitrates so if you do not remove the nitrates from the water you still have algae food and we really do not test for nitrates as a matter of course. Why? Because to remove nitrates you have to replace the water with nitrate free water or shock with VERY HIGH chlorine levels (read that as nuke the pool!) and that does not always work. There is no magic elixer that can be sold to us to remove nitrates like there is with phosphates so they just ignore them!

 

[chem geek]

Both nitrates (or ammonia or nitrites as a source of nitrogen; nitrogen gas for nitrogen-fixing cyanobacteria) and phosphates (as a source of phosphorus) are essential nutrients for algae, along with carbonates (including carbon dioxide as a source of carbon) and water (as a source of hydrogen and oxygen). The ratio of nitrogen to phosphorus influences the kind of algae that are favored for growth where a low N ratio as well as a limiting nitrate supply favors blue-green (nitrogen-fixing) algae (technically, cyanobacteria) whereas a high N ratio favors green algae.

In theory, if the phosphate level gets extremely low, then algae cannot grow quickly since there is no alternate source for phosphorus. Those that sell phosphate removers claim that 100 or 150 ppb is that limiting level and in most cases pools with algae have had the algae inhibited when phosphates were reduced to such low levels. On the other hand, we've seen at least one report where a phosphate remover did not stop the algae growth. It should be noted that lanthanum chloride which is used to precipitate lanthanum phosphate to remove phosphates, will only remove the inorganic phosphate and will not remove organic phosphates some of which can still be used as a nutrient for algae. It should also be noted that the phosphate tests only test for inorganic phosphate (aka orthophosphate).

Since one has to continually remove phosphates to keep their level low, the maintenance process, after the initial expensive large phosphate reduction, is similar to using a regular algaecide. It's extra cost and can be seen as insurance (i.e. optional, but not necessary) to lessen the chance for fast algae growth in case the chlorine level (FC/CYA ratio) gets too low. This assumes that the primary source of phosphates in the water are inorganic phosphates and not bioavailable organic phosphates.

Richard